The invention relates to an endoprosthetic knee joint with a femur portion having, at the lower end of a femur shaft, convexly curved condylar shells with first sliding surfaces which have dorsally two spaced-apart side walls forming an intercondylar space, with a tibia portion having at the upper end of a tibia shaft a tibia plateau upon which there are formed secondary sliding surfaces corresponding to the first sliding surfaces of the condylar shells, and which has an aperture extending axially in the tibia shaft, and with a coupling portion having, at the upper end of a coupling pin accommodated rotatably in the aperture, a joint head projecting into the intercondylar space, said joint head having a bore extending transversely to the femur axis, and being pivotally mounted by means of a coupling bolt passing through the bore and supported on the adjacent side walls of the condylar shells.
Such an endoprosthetic knee joint with a coupling portion in the form of a pure hinge joint is known from DE 35 29 894 C2. The disadvantage of such an endoprosthetic knee joint resides particularly in the fact that, when there are axial faulty positionings between the femur and the tibia, no varus/valgus compensation is possible and the resultant one-sided load must be taken up totally by the coupling bolt or its anchorage in the side walls. As it is in many cases conventional for reasons of weight-saving to connect the two side walls of the condylar shells by means of a web to form a box, upon which the femur shaft is then set, box breakages with a lack of possibility for varus/valgus compensation are well-known consequences. This problem was recognised at an early stage, and the rigid coupling bolt was replaced by a sphere which is attached to the point of a stem emerging from the tibia portion and accommodated in a corresponding bowl in the box of the femur portion after the fashion of a joint head. Such an endoprosthetic knee joint is known from EP 0 639 358 or from GB-A-2,088,724. The problem in such a spherical construction resides in suspending the sphere in the bowl and in the latent risk of dislocation in the case of traction or extreme movement. The risk of dislocation is intended to be partly counteracted by the fact that the stem upon whose point the sphere is attached was not rigidly connected to the tibia portion, but was let into a recess in the tibia shaft, whereupon in the case of tension stress, the ball joint was not drawn out of the bowl, but the stem out of the recess, which is of little importance due to the long distance covered.
There is known from DE-OS 41 02 509 a more recent construction of an endoprosthetic knee joint with a hinge joint, in which a stationary sphere on a stem and a slotted bowl secured to the femur portion are provided. The problem of suspension and of dislocation becomes clear with reference to this known construction.
Finally, a further problem arises in all these spherical constructions, from the fact that the most important forces acting on the coupling bolt are oscillating, anteriorposterior thrust forces, which cannot be neutralised, or only to a slight degree, by the lateral hemispheres articulated to the box wall, so that the supporting surfaces on the central portions of the sphere or of the ball are reduced and consequently there is a risk of overloading.